The UHP Western Gneiss Region, Norway, part IV: The garnets
Last Updated: 5th Jan 2023By Olav Revheim
This is the fourth article in a series of six on the Norwegian Western Gneiss Region. The articles are:
• The UHP Western Gneiss Region, Norway, part I: Geologic setting
• The UHP Western Gneiss Region, Norway, part II: Eclogite facies HP/UHP rocks
• The UHP Western Gneiss Region, Norway, part III: Some eclogite localities
• The UHP Western Gneiss Region, Norway, part IV: The garnets
• The UHP Western Gneiss Region, Norway, part V: The pyroxenes
• The UHP Western Gneiss Region, Norway, part VI: The amphiboles
The Western Gneiss Region (WGR) is a metamorphic terrain located along the coast between the cities of Bergen and Trondheim in Norway. It occupies an area of approximately 50.000 km2.
The dominant rock types are migmatitic orthogneisses of tonalitic or granitic composition. Within these gneisses lesser amounts of metasedimentary rocks (paragneisses, marbles and quartzites), anorthosites, augen gneisses, gabbros and peridotites can be found.
There are hundreds of smaller or larger lenses, boudins, pods and bodies of eclogite facies rocks scattered throughout this large area. Eclogite facies mineralogies are found as bodies, lenses, boudins and/or layers in all the different rock types. Garnet is a key component of many of the amphibole and eclogite facies rocks in this domain.
The high-grade metamorphic garnets rarely show well defined crystal faces, as can be found in schists and pegmatites. Instead, they can form nicely colored specimens when the red garnets occur with bright green pyroxenes but the garnets in the WGR are mostly interesting from a petrological and geological perspective. The garnets are stable in both amphibolite, granulite and eclogite facies conditions and the garnets, their inclusions and alteration products are important contributors in our understanding of both prograde, peak and retrograde metamorphic processes and conditions in this region.
Garnets are found as a rock forming mineral in most of the eclogite facies rocks such as eclogite, garnet websterite and garnet peridotites and in many amphibolite facies rocks, such as gneisses and amphibolites. Like the majority of research, this article will focus on the eclogite facies garnets.
In the eclogite facies rocks, garnets are most frequently found as rounded crystals or porphyroblasts in a pyroxene (omphacite, enstatite and/or diopside) groundmass with or without olivine (forsterite). These porphyroblasts range in size from a few mm up to 25cm or more. Many of the garnet porphyroblasts were formed earlier that the Scandian eclogite facies event and have retained evidence of the changing conditions before and during the Scandian Orogeny. Early formed garnets may have overgrowth of re-crystallized garnets resulting in zoned crystals, where each zone represents a separate metamorphic or igneous event.
More rarely, garnets occur as lenses or layers in the rocks (granitites) or as thin, late forming veins. These veins have most often been formed under eclogite facies conditions.
At a few localities, the protoliths of the eclogite facies rocks has remained unaltered despite that the rock has seen high to ultra-high pressures outside the stability conditions of these rocks. Both Mørk (1985, 1986) and Wain et al. (2001) describe the initial stages of eclogitization.
Straume and Austrheim (1999) also describe the initial amphibolite facies retrograde breakdown of garnet into plagioclase and amphibole. This alteration is first visible in fractures and grain boundaries within the garnet and then on further breakdown a symplectitic corona appears around the garnet grains.
All garnets found in the eclogite facies rocks in the WGR are in the almandine-pyrope series with a variable grossular component. Mørk (1986) found the grossular component to de dominant in garnet coronas around plagioclase feldspar, but that seems to be an anomaly. Most of the published analyses falls in the almandine space, some in the pyrope space and several analyses show compositions near the border between almandine and pyrope. It is nevertheless possible to reliably identify a garnet based on the associated minerals and whether it is found in a rock of “internal” or “external” origin (e.g. locality).
The garnets found in “internal” rocks originating from mantle wedge peridotites are invariably pyropes, whereas the garnets originating from “external” crustal rocks are predominantly almandine. The only “external” rock-type carrying pyrope are the garnet peridotites. This leads to a general rule of thumb in naming garnets from the WGR; pyrope is found in rocks with a high forsterite (olivine) content and in rocks associated with “internal” peridotites. All other garnets are almandine.
Garnets reflects the geochemical environment they were formed in, and they often show zoning or a clear compositional development from the core to the rim, reflecting changes in local chemical compositions or PT conditions during their formation. This is also the case for many garnets in the WGR. Since garnets is stable in a large range of PT conditions and many of the rocks in the WGR, changes in garnet compositions have been used to decipher the metamorphic history of the region. (Martin, Baxter 2017, Simakov 2008, Medaris et al., 2018 and others).
As the examples above show, the garnet compositions is a function of the eclogite protolith and the metamorphic grade. It appears that a more detailed analysis of garnet compositions across the WGR would provide more insights in the regional and local metamorphism in the WGR.
Garnet crystals often contain inclusions of other minerals. These inclusions often shed further light on the metamorphic history of the rock, as the garnet acts as a pressure vessel protecting the inclusions from changing pressure conditions and external chemical influences.
Garnet from the “external” eclogites often contains inclusions that shows the prograde evolution of the eclogites, but the actual prograde PT paths are rarely described. Jamtveit (1987) give a detailed account of the inclusions in large garnets in the eclogite facies rocks from Eiksunddal. The garnets are usually very rich in small inclusion of pyroxene, amphibole and occasionally phlogopite. The inclusions are usually enriched in the core.
The amphiboles show compositional changes reflecting the metamorphic conditions when they formed. Early magnesio-hornblende/pargasite is assumed to be pre-eclogite-facies, whereas edenite/magnesio-katophorite compositions is interpreted to have formed during the eclogite facies event. A third generation, also magnesio-hornblende/pargasite occurs as rims on pyroxene. They clearly post-date the eclogite facies event. It appears that these have formed where fluid has penetrated the garnet crystals in micro-cracks during the uplift of the rocks.
Additional eclogite facies inclusions in garnets are found in Jamtveit’s (1987) “sodic-amphibolites” e.g., retrograded eclogites. The eclogite facies inclusions that are preserved in the consist of jadeite-rich clinopyroxene, kyanite, quartz, rutile and phengite. All of these minerals are also common inclusions in garnets from other eclogite facies rocks in the WGR.
Quartz is one of the minerals that are frequently found as inclusions in eclogite garnets. Quartz occurs both as minute (up to mm size) grains and as polycrystalline aggregates. The latter is of particular interest because they are interpreted as pseudomorphs of quartz after coesite. Coesite is the stable SiO2 polymorph at ultra-high pressures (UHP). The presence of quartz pseudomorphs after coesite is a very important geological marker and represent the border between HP and UHP conditions
Coesite rarely survives the uplift of UHP rocks, and the former presence of coesite is normally only evidenced by minute polycrystalline quartz aggregates (Mosenfelder et al., 2005 and Carswell et al., 2003). These polycrystalline quartz aggregates are taken as evidence that the rocks have seen ultra-high pressure, and such aggregates has been found as inclusions in both “internal” and “external” garnets. It is predominantly the presence of quartz pseudomorphs after coesite in garnets that are used to differentiate between HP and UHP eclogite facies in WGR.
Micro-diamonds from the WGR were first described by Dobrzhinetskaya et al. (1995). They positively identified three diamond crystals of 10–20 µm and 45 µm in diameter from a garnet-kyanite-phlogopite gneiss from Fjørtoft. This discovery evidenced that the UHP conditions of the WGR locally reached the diamond stability field, corresponding to subduction depths of 110-120km.
Van Roermund et al. (2002) identified diamonds (typically ca 5 μm in size) as inclusions in Cr-bearing spinel inclusion in garnet coronas around orthopyroxene (enstatite) megacrysts from a garnet-websterite pod, also on Fjørtoft, where they occur with Ti-phlogopite, kalsilite, magnesite, dolomite, Ba-Mg carbonate, Fe-Ni sulfide, Cl-apatite, rutile, zircon and monazite.
Vrijmoed et al. (2006) identified diamonds in polyphase inclusions in garnets from garnet-websterite veins cross-cutting a Fe-Ti peridotite from the Svartberget external eclogite-
The origin of the diamonds have excessively discussed, but the most accepted theory suggest that they were formed during the Scandian Orogeny (410.6 ± 2.6 270 Ma, Quas-Cohen, 2013), also those found in the mantle derived “internal” garnet peridotites. The diamonds were formed by the infiltration of a super-critical C-O-H rich fluid from a crustal origin during the UHP metamorphic event.
In high pressure and temperature conditions, many minerals have a slightly different chemistry than they do at lower pressures and temperatures. At extreme PT conditions, pyrope will become more silicic, introducing a minor majorite (Mg3(MgSi)(SiO4)3 component. During uplift of the rocks, pyrope can no longer contain the majoritic component, and this is exsolved as pyroxenes.
Van Roermund and Drury (1998) and Van Roermund et al. (2001) describes both clinopyroxene and orthopyroxene exsolution needles in some of the garnet grain cores in garnet peridotites on Otrøy.
Van Reormund et al. (2017) considers that pyroxene exsolution from garnet occurred in two “steps”:
1. an intercrystalline stage resulting in “precipitation” (and growth) of interstitial mm-scale pyoxene grains in between adjacent garnet grains having clear precipitation free rims and
2. intracrystalline μm-scale pyroxene exsolution
By calculating the Al partitioning between pyrope and pyroxene, the PT conditions and thereby a maximum subduction depth of more than 185 km can be calculated. Later, pyroxene exsolutions in garnets have been found in several of the UHP garnet peridotites and van Roermund et al. (2017) give an overview of localities, literature, and interpretation of these finds.
Garnets are stable in almost the entire pressure and temperature range seen in the WGR, from the Sveco-Norwegian amphibolite/granulite facies rocks, via the HP and UHP eclogite facies rocks to the retrograde amphibolite facies rocks. Even at mantle conditions garnet crystallizes. There are still several examples of garnet breakdown/re-crystallization during the retrograde amphibolite facies event. During this event most of the eclogites formed amphibolite zones towards the host gneisses and many eclogites were completely altered to amphibolites, There are also partial retrograde alteration of eclogites. Both Wain et al. (2001) and Straume and Austrheim (1999) describe the eclogite retrogression process.
The retrograde alteration starts in fractures in the garnets, and symplectitic intergrowths of orthopyroxene-plagioclase and spinel fill the fractures within and around the garnet grains. Straume and Austrheim (1999) describe two breakdown reactions:
1) Grt1+Cpx=Grt2+Spl+Opx+Pl
2) Qtz+Grt1=Opx+Pl+Grt2
The eclogite garnet (Grt1) is typically an almandine -pyrope series garnets with a significant grossular component. Garnet 2 is an amphibolite/granulite facies garnet which contains much less of the pyrope and grossular components, e.g. a much closer to the almandine end-member composition consistent with amphibolite facies conditions.
The “internal” peridotites are often evident on surface as chlorite-peridotites with forsterite and Cr-bearing chlorite as main minerals. Cuthbert and van Roermund (2008) consider that the chlorite peridotites have formed from garnet-peridotites via retrograde alteration of pyrope to chlorites. Apeiranthiti (2016) describes intergrowths of fine-grained, fibrous orthopyroxenes and clinopyroxenes as a kelyphitic corona around garnet grains from Kvalvika. The kelyphites are often rimmed by amphibole. The kelyphites are interpreted to have formed at an early retrograde stage, but the PT conditions has not been calculated due to difficulties in accurately analyze the fine-grained pyroxene fibers, although eclogite facies conditions seem likely.
The WGR eclogite facies rocks have either a mantle (“internal”) or a crustal (“external”) origin. Garnet compositions has been and is instrumental in differentiating between similar looking rocks of different origin and changes in garnet composition, the inclusions in garnets and the garnet breakdown products are all important for understanding the geological history of the HP/UHP rocks of the WGR.
The pyropes of the “internal” rocks are studies to better understand their geological origins and the processes involved in embedding these rock bodies into the subducted crust. In particular the majoritic garnets have been important in this work.
Van Roermund et al. (2017) describe three generations of garnets from UHP “internal eclogites. The original composition of these garnets is calculated from the bulk chemistry of the garnets and pyroxene exsolution micro-structures. Differences in garnet chemistry and inclusions has led to the acknowledgement that there were 3 main garnet forming events for the pyropes in the “internal” garnet peridotites and eclogites:
An M1 event (2600-2800Ma) formed majoritic garnets (5-8 to perhaps as much as 20%) interpreted to have originated within uprising mantle from depths of at least 185 km
An M2 event (1650-1400Ma) exsolved garnets and pyroxenes from M1 garnets
An M3 event (430Ma) during maximum PT condition during Scandian subduction. At some localities in the northwestern WGR, the M3 garnets have a small (ca 1%) majoritic component. It is also in this event that the micro-diamonds formed.
The garnet composition does not only aid in differentiating between “internal” and “external” rocks. The composition of different zones reflects different crystallization events. The PT paths in the HP domains and the retrograde alteration between peak metamorphism and the large amphibolite facies event is little studied, and a prograde path for the ” external” eclogites has not been established. Carswell et al. (1983) describe zoned garnets with different inclusions from different events from Årsheimneset. They do not provide any data or elaboration on this prograde zoning but find a UHP garnet overgrowth with quartz pseudomorphs after coesite in parts of the eclogite that has been exposed to UHP fluids. The inclusions in the garnets from Årsheimneset illustrate that some eclogites have seen both HP and UHP conditions.
An interesting aspect, and perhaps a unique feature of the WGR is the partial transition from “external” crustal rocks to eclogite and from eclogites to retrograde amphibolites. This gives valuable insight into the eclogitization process. Both Mørk (1985,1986) and Wain et al. (2001) have studied rocks with a beginning or partial eclogitization.
Mørk (1985, 1986 &1988) studied the gabbros on Flemsøy, where she finds both unaltered gabbro, transitional stages between gabbro and eclogite, eclogites and retrograded eclogites (amphibolites) originating from the gabbro. The eclogitization process is initiated by the formation of garnets in the grain boundaries of the plagioclase and the pyroxene in the gabbro. In more eclogitized gabbros, orthopyroxene replaces olivine and omphacite starts to appear on other pyroxene grains. Garnet replaces plagioclase.
Wain et al. (2001) describe the eclogitization of a granulite in the Flatraket Complex. Here, the granulite is cut by a 2 cm thick eclogite band where the granulite shows transitional stages form the eclogite and up to 15mm into the granulite. Wain et al. (2001) shows a clear chemical difference between Sweco-Norwegian granulite facies and Scandian eclogite facies garnet compositions.
The composition of the garnets from the WGR can be used to differentiate between rocks of “internal” and “external” origin.
The composition and inclusions in garnets are also important in the efforts to reconstruct the PT paths and metamorphic events for both “internal” and “external” garnets.
The majoritic component of some “internal” garnets show that the “internal” rocks originate deep in the mantle. In the northwestern part of the WGR, also some of the Scandian garnets have seen pressures giving a small (~1% majorite) component thereby showing the maximum subduction depths.
Garnets formed from meta-stable gabbros and granulites illustrates the early stages of eclogitization of crustal rocks. They also show that rocks may retain their mineralogy well outside their stability area. This may also be the case for the HP garnets in the HP/UHP Årsheimneset eclogite.
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• The UHP Western Gneiss Region, Norway, part I: Geologic setting
• The UHP Western Gneiss Region, Norway, part II: Eclogite facies HP/UHP rocks
• The UHP Western Gneiss Region, Norway, part III: Some eclogite localities
• The UHP Western Gneiss Region, Norway, part IV: The garnets
• The UHP Western Gneiss Region, Norway, part V: The pyroxenes
• The UHP Western Gneiss Region, Norway, part VI: The amphiboles
Introduction
The Western Gneiss Region (WGR) is a metamorphic terrain located along the coast between the cities of Bergen and Trondheim in Norway. It occupies an area of approximately 50.000 km2.
The dominant rock types are migmatitic orthogneisses of tonalitic or granitic composition. Within these gneisses lesser amounts of metasedimentary rocks (paragneisses, marbles and quartzites), anorthosites, augen gneisses, gabbros and peridotites can be found.
Some WGR eclogite localities
There are hundreds of smaller or larger lenses, boudins, pods and bodies of eclogite facies rocks scattered throughout this large area. Eclogite facies mineralogies are found as bodies, lenses, boudins and/or layers in all the different rock types. Garnet is a key component of many of the amphibole and eclogite facies rocks in this domain.
The high-grade metamorphic garnets rarely show well defined crystal faces, as can be found in schists and pegmatites. Instead, they can form nicely colored specimens when the red garnets occur with bright green pyroxenes but the garnets in the WGR are mostly interesting from a petrological and geological perspective. The garnets are stable in both amphibolite, granulite and eclogite facies conditions and the garnets, their inclusions and alteration products are important contributors in our understanding of both prograde, peak and retrograde metamorphic processes and conditions in this region.
Type of occurrences
Garnets are found as a rock forming mineral in most of the eclogite facies rocks such as eclogite, garnet websterite and garnet peridotites and in many amphibolite facies rocks, such as gneisses and amphibolites. Like the majority of research, this article will focus on the eclogite facies garnets.
In the eclogite facies rocks, garnets are most frequently found as rounded crystals or porphyroblasts in a pyroxene (omphacite, enstatite and/or diopside) groundmass with or without olivine (forsterite). These porphyroblasts range in size from a few mm up to 25cm or more. Many of the garnet porphyroblasts were formed earlier that the Scandian eclogite facies event and have retained evidence of the changing conditions before and during the Scandian Orogeny. Early formed garnets may have overgrowth of re-crystallized garnets resulting in zoned crystals, where each zone represents a separate metamorphic or igneous event.
More rarely, garnets occur as lenses or layers in the rocks (granitites) or as thin, late forming veins. These veins have most often been formed under eclogite facies conditions.
At a few localities, the protoliths of the eclogite facies rocks has remained unaltered despite that the rock has seen high to ultra-high pressures outside the stability conditions of these rocks. Both Mørk (1985, 1986) and Wain et al. (2001) describe the initial stages of eclogitization.
Straume and Austrheim (1999) also describe the initial amphibolite facies retrograde breakdown of garnet into plagioclase and amphibole. This alteration is first visible in fractures and grain boundaries within the garnet and then on further breakdown a symplectitic corona appears around the garnet grains.
Garnet compositions.
All garnets found in the eclogite facies rocks in the WGR are in the almandine-pyrope series with a variable grossular component. Mørk (1986) found the grossular component to de dominant in garnet coronas around plagioclase feldspar, but that seems to be an anomaly. Most of the published analyses falls in the almandine space, some in the pyrope space and several analyses show compositions near the border between almandine and pyrope. It is nevertheless possible to reliably identify a garnet based on the associated minerals and whether it is found in a rock of “internal” or “external” origin (e.g. locality).
The different chemistry between garnets of "internal" mantle origin and "external" crustal origin is obvious in this Mg/(Mg+Fe) vs Cr plot, with the "internal" rocks richer in Mg and Cr.
The garnets found in “internal” rocks originating from mantle wedge peridotites are invariably pyropes, whereas the garnets originating from “external” crustal rocks are predominantly almandine. The only “external” rock-type carrying pyrope are the garnet peridotites. This leads to a general rule of thumb in naming garnets from the WGR; pyrope is found in rocks with a high forsterite (olivine) content and in rocks associated with “internal” peridotites. All other garnets are almandine.
Garnets reflects the geochemical environment they were formed in, and they often show zoning or a clear compositional development from the core to the rim, reflecting changes in local chemical compositions or PT conditions during their formation. This is also the case for many garnets in the WGR. Since garnets is stable in a large range of PT conditions and many of the rocks in the WGR, changes in garnet compositions have been used to decipher the metamorphic history of the region. (Martin, Baxter 2017, Simakov 2008, Medaris et al., 2018 and others).
Analyses ( Wain et al., 2001) showing garnet zoning as a function of metamorphic evolution from the Flatraket body. The leftmost chart show a granulite facies garnet with an eclogite facies outer zone. The eclogite facies garnet are characterized by a higher grossular (Ca) component than the eclogite facies garnet. The middle analysis also show a granulite facies garnet with an outer zone of Ca-rich almandine (eclogite facies). In addition, this garnet has an amphibolite facies rim characterized by a higher Fe and lower Mg content than the eclogite facies garnet. The leftmost analysis show a corona garnet that show a beginning eclogitization of granulites. The garnet has formed between garnet and quartz grains, and show a much lower Mg content near plagioclase.
As the examples above show, the garnet compositions is a function of the eclogite protolith and the metamorphic grade. It appears that a more detailed analysis of garnet compositions across the WGR would provide more insights in the regional and local metamorphism in the WGR.
Inclusions in garnets
Garnet crystals often contain inclusions of other minerals. These inclusions often shed further light on the metamorphic history of the rock, as the garnet acts as a pressure vessel protecting the inclusions from changing pressure conditions and external chemical influences.
Amphiboles (and pyroxenes)
Garnet from the “external” eclogites often contains inclusions that shows the prograde evolution of the eclogites, but the actual prograde PT paths are rarely described. Jamtveit (1987) give a detailed account of the inclusions in large garnets in the eclogite facies rocks from Eiksunddal. The garnets are usually very rich in small inclusion of pyroxene, amphibole and occasionally phlogopite. The inclusions are usually enriched in the core.
The amphiboles show compositional changes reflecting the metamorphic conditions when they formed. Early magnesio-hornblende/pargasite is assumed to be pre-eclogite-facies, whereas edenite/magnesio-katophorite compositions is interpreted to have formed during the eclogite facies event. A third generation, also magnesio-hornblende/pargasite occurs as rims on pyroxene. They clearly post-date the eclogite facies event. It appears that these have formed where fluid has penetrated the garnet crystals in micro-cracks during the uplift of the rocks.
Additional eclogite facies inclusions in garnets are found in Jamtveit’s (1987) “sodic-amphibolites” e.g., retrograded eclogites. The eclogite facies inclusions that are preserved in the consist of jadeite-rich clinopyroxene, kyanite, quartz, rutile and phengite. All of these minerals are also common inclusions in garnets from other eclogite facies rocks in the WGR.
Quartz/Coesite – evidence of ultra-high pressure
Quartz is one of the minerals that are frequently found as inclusions in eclogite garnets. Quartz occurs both as minute (up to mm size) grains and as polycrystalline aggregates. The latter is of particular interest because they are interpreted as pseudomorphs of quartz after coesite. Coesite is the stable SiO2 polymorph at ultra-high pressures (UHP). The presence of quartz pseudomorphs after coesite is a very important geological marker and represent the border between HP and UHP conditions
Coesite rarely survives the uplift of UHP rocks, and the former presence of coesite is normally only evidenced by minute polycrystalline quartz aggregates (Mosenfelder et al., 2005 and Carswell et al., 2003). These polycrystalline quartz aggregates are taken as evidence that the rocks have seen ultra-high pressure, and such aggregates has been found as inclusions in both “internal” and “external” garnets. It is predominantly the presence of quartz pseudomorphs after coesite in garnets that are used to differentiate between HP and UHP eclogite facies in WGR.
Micro-diamonds
Micro-diamonds from the WGR were first described by Dobrzhinetskaya et al. (1995). They positively identified three diamond crystals of 10–20 µm and 45 µm in diameter from a garnet-kyanite-phlogopite gneiss from Fjørtoft. This discovery evidenced that the UHP conditions of the WGR locally reached the diamond stability field, corresponding to subduction depths of 110-120km.
Van Roermund et al. (2002) identified diamonds (typically ca 5 μm in size) as inclusions in Cr-bearing spinel inclusion in garnet coronas around orthopyroxene (enstatite) megacrysts from a garnet-websterite pod, also on Fjørtoft, where they occur with Ti-phlogopite, kalsilite, magnesite, dolomite, Ba-Mg carbonate, Fe-Ni sulfide, Cl-apatite, rutile, zircon and monazite.
Vrijmoed et al. (2006) identified diamonds in polyphase inclusions in garnets from garnet-websterite veins cross-cutting a Fe-Ti peridotite from the Svartberget external eclogite-
The origin of the diamonds have excessively discussed, but the most accepted theory suggest that they were formed during the Scandian Orogeny (410.6 ± 2.6 270 Ma, Quas-Cohen, 2013), also those found in the mantle derived “internal” garnet peridotites. The diamonds were formed by the infiltration of a super-critical C-O-H rich fluid from a crustal origin during the UHP metamorphic event.
Pyroxene exsolutions
In high pressure and temperature conditions, many minerals have a slightly different chemistry than they do at lower pressures and temperatures. At extreme PT conditions, pyrope will become more silicic, introducing a minor majorite (Mg3(MgSi)(SiO4)3 component. During uplift of the rocks, pyrope can no longer contain the majoritic component, and this is exsolved as pyroxenes.
Van Roermund and Drury (1998) and Van Roermund et al. (2001) describes both clinopyroxene and orthopyroxene exsolution needles in some of the garnet grain cores in garnet peridotites on Otrøy.
Van Reormund et al. (2017) considers that pyroxene exsolution from garnet occurred in two “steps”:
1. an intercrystalline stage resulting in “precipitation” (and growth) of interstitial mm-scale pyoxene grains in between adjacent garnet grains having clear precipitation free rims and
2. intracrystalline μm-scale pyroxene exsolution
By calculating the Al partitioning between pyrope and pyroxene, the PT conditions and thereby a maximum subduction depth of more than 185 km can be calculated. Later, pyroxene exsolutions in garnets have been found in several of the UHP garnet peridotites and van Roermund et al. (2017) give an overview of localities, literature, and interpretation of these finds.
Breakdown of garnets.
Garnets are stable in almost the entire pressure and temperature range seen in the WGR, from the Sveco-Norwegian amphibolite/granulite facies rocks, via the HP and UHP eclogite facies rocks to the retrograde amphibolite facies rocks. Even at mantle conditions garnet crystallizes. There are still several examples of garnet breakdown/re-crystallization during the retrograde amphibolite facies event. During this event most of the eclogites formed amphibolite zones towards the host gneisses and many eclogites were completely altered to amphibolites, There are also partial retrograde alteration of eclogites. Both Wain et al. (2001) and Straume and Austrheim (1999) describe the eclogite retrogression process.
The retrograde alteration starts in fractures in the garnets, and symplectitic intergrowths of orthopyroxene-plagioclase and spinel fill the fractures within and around the garnet grains. Straume and Austrheim (1999) describe two breakdown reactions:
1) Grt1+Cpx=Grt2+Spl+Opx+Pl
2) Qtz+Grt1=Opx+Pl+Grt2
The eclogite garnet (Grt1) is typically an almandine -pyrope series garnets with a significant grossular component. Garnet 2 is an amphibolite/granulite facies garnet which contains much less of the pyrope and grossular components, e.g. a much closer to the almandine end-member composition consistent with amphibolite facies conditions.
The “internal” peridotites are often evident on surface as chlorite-peridotites with forsterite and Cr-bearing chlorite as main minerals. Cuthbert and van Roermund (2008) consider that the chlorite peridotites have formed from garnet-peridotites via retrograde alteration of pyrope to chlorites. Apeiranthiti (2016) describes intergrowths of fine-grained, fibrous orthopyroxenes and clinopyroxenes as a kelyphitic corona around garnet grains from Kvalvika. The kelyphites are often rimmed by amphibole. The kelyphites are interpreted to have formed at an early retrograde stage, but the PT conditions has not been calculated due to difficulties in accurately analyze the fine-grained pyroxene fibers, although eclogite facies conditions seem likely.
Significance of garnets in the understanding of WGR geology
The WGR eclogite facies rocks have either a mantle (“internal”) or a crustal (“external”) origin. Garnet compositions has been and is instrumental in differentiating between similar looking rocks of different origin and changes in garnet composition, the inclusions in garnets and the garnet breakdown products are all important for understanding the geological history of the HP/UHP rocks of the WGR.
Internal rocks
The pyropes of the “internal” rocks are studies to better understand their geological origins and the processes involved in embedding these rock bodies into the subducted crust. In particular the majoritic garnets have been important in this work.
Van Roermund et al. (2017) describe three generations of garnets from UHP “internal eclogites. The original composition of these garnets is calculated from the bulk chemistry of the garnets and pyroxene exsolution micro-structures. Differences in garnet chemistry and inclusions has led to the acknowledgement that there were 3 main garnet forming events for the pyropes in the “internal” garnet peridotites and eclogites:
An M1 event (2600-2800Ma) formed majoritic garnets (5-8 to perhaps as much as 20%) interpreted to have originated within uprising mantle from depths of at least 185 km
An M2 event (1650-1400Ma) exsolved garnets and pyroxenes from M1 garnets
An M3 event (430Ma) during maximum PT condition during Scandian subduction. At some localities in the northwestern WGR, the M3 garnets have a small (ca 1%) majoritic component. It is also in this event that the micro-diamonds formed.
The figure show an approximate PT evolution of the «internal» garnet peridotites from the deepest subducted rocks in the WGR, Modified after Van Reormund et al. (2017)
“External” rocks
The garnet composition does not only aid in differentiating between “internal” and “external” rocks. The composition of different zones reflects different crystallization events. The PT paths in the HP domains and the retrograde alteration between peak metamorphism and the large amphibolite facies event is little studied, and a prograde path for the ” external” eclogites has not been established. Carswell et al. (1983) describe zoned garnets with different inclusions from different events from Årsheimneset. They do not provide any data or elaboration on this prograde zoning but find a UHP garnet overgrowth with quartz pseudomorphs after coesite in parts of the eclogite that has been exposed to UHP fluids. The inclusions in the garnets from Årsheimneset illustrate that some eclogites have seen both HP and UHP conditions.
An interesting aspect, and perhaps a unique feature of the WGR is the partial transition from “external” crustal rocks to eclogite and from eclogites to retrograde amphibolites. This gives valuable insight into the eclogitization process. Both Mørk (1985,1986) and Wain et al. (2001) have studied rocks with a beginning or partial eclogitization.
Mørk (1985, 1986 &1988) studied the gabbros on Flemsøy, where she finds both unaltered gabbro, transitional stages between gabbro and eclogite, eclogites and retrograded eclogites (amphibolites) originating from the gabbro. The eclogitization process is initiated by the formation of garnets in the grain boundaries of the plagioclase and the pyroxene in the gabbro. In more eclogitized gabbros, orthopyroxene replaces olivine and omphacite starts to appear on other pyroxene grains. Garnet replaces plagioclase.
Wain et al. (2001) describe the eclogitization of a granulite in the Flatraket Complex. Here, the granulite is cut by a 2 cm thick eclogite band where the granulite shows transitional stages form the eclogite and up to 15mm into the granulite. Wain et al. (2001) shows a clear chemical difference between Sweco-Norwegian granulite facies and Scandian eclogite facies garnet compositions.
Conclusions
The composition of the garnets from the WGR can be used to differentiate between rocks of “internal” and “external” origin.
The composition and inclusions in garnets are also important in the efforts to reconstruct the PT paths and metamorphic events for both “internal” and “external” garnets.
The majoritic component of some “internal” garnets show that the “internal” rocks originate deep in the mantle. In the northwestern part of the WGR, also some of the Scandian garnets have seen pressures giving a small (~1% majorite) component thereby showing the maximum subduction depths.
Garnets formed from meta-stable gabbros and granulites illustrates the early stages of eclogitization of crustal rocks. They also show that rocks may retain their mineralogy well outside their stability area. This may also be the case for the HP garnets in the HP/UHP Årsheimneset eclogite.
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